test latest version rather than main

Author: galabovaa

.github/workflows/ortools-ubuntu.yml

@@ -36,7 +36,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: google/or-tools
-          ref: 9789365f574f5e4857117d410ff177bbbae622fd
+          ref: v9.15
           path: or-tools
 
       - name: LS